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Heat Exchangers Help Reduce Climate Change

Heat exchangers allow you to save significant energy and thus also money. In doing so they also make a real contribution towards climate change mitigation.

EU Commission: Immense CO2 Savings Potential

Worldwide, heat exchangers have an enormous potential to reduce CO2 emissions. According to a working document of the European Commission from 2012, some 100 million tonnes of CO2 are currently saved each year in the EU alone as a result of the use of heat recovery systems. And this applies only to non-residential buildings - i.e. offices, factories, hospitals, schools etc. The savings provided from residential buildings are therefore additional.

The study further indicates that with increased heat recovery, a further 50 million tonnes of CO2 can be spared per year by 2025 compared to the 2010 figure.

Case Study

Anyone who has an air handling unit in their building can effectively contribute to climate change mitigation. The extent can be illustrated with the example of a rotary heat exchanger just under three metres in diameter. Under prevailing climatic conditions in Western Europe, such rotors normally recover some 291,000 kilowatt-hours (kWh) of heat per year (see design calculation) in normal operation.

According to the report of the Enquete commission titled ‘Protection of the Earth’s AtmosPWTre’ conducted by the German Bundestag in 1994, CO2 emissions from using fossil fuels (lignite, anthracite, mineral oil and natural gas) can be estimated as follows:

CO2 Emissions by Energy Source

Energy Source

CO2 emission (kg) per kilowatt-hour

Lignite

0,400 kg CO2 / kWh

Anthracite

0,330 kg CO2 / kWh

Mineral oil

0,262 kg CO2 / kWh

Natural gas

0,199 kg CO2 / kWh

The CO2 savings potential for our rotary heat exchanger can be calculated based on these figures. Depending on which of these four energy sources are used, the following annual reductions in CO2 emissions are obtained (figures rounded):

CO2 Reduction using Rotary Heat Exchangers

Energy Sourse

Reductions in tons per year

Lignite

116,5 t/a

Anthracite

96,1 t/a

Mineral oil

76,3 t/a

Natural gas

58,0 t/a

Below we would like to demonstrate how these results are calculated. For this, however, we must first perform the technical design of the rotary heat exchanger and then calculate the viability.

Design of a Rotary Heat Exchanger

Rotor type: RRS-P-C16-3000/3000-2840

Foil thickness 0,08 mm

Wave height 1,6 mm

Standard volume ( 20 °C / 50 % / 1013 mbar )

Volumes of supply and exhaust air approx. 30,000 m³/h

Air intake

Supply air

Exhaust air

Temperature

-12 °C

22 °C

Air volume

26628 m³/h

30192 m³/h

Rel. humidity

90 %

40 %

Abs. humidity

1,20 g/kg

6,56 g/kg

Air discharge

Supply air

Exhaust air

Temperature

14,0 °C

-2,5 °C

Air volume

29424 m³/h

27527 m³/h

Rel. humidity

43 %

95 %

Abs. humidity

4,21 g/kg

2,91 g/kg

From these figures we obtain heat exchange efficiencies for this rotor of 77% for the supply air and 71% for the exhaust air. The rotor also transfers 56% of the moisture of the supply air and 68% of that of the exhaust air.

Exchange Capacities

Sensible heat

263 kW

Latent heat

75 kW

Total heat

338 kW

Humidity recovery

108 kg/h

Viability Calculation

To determine the potential for CO2 reduction we use the following viability calculation for winter operation. This is based on the specifications of the VDI 2067 guideline (Viability of building services equipment).

Winter operation is assumed to occur between November and April, i.e. the months in which buildings are heated in Climate Zone 3 countries (including Germany).

The design is valid for an average air pressure of 1013 millibars. The system operating hours are based on six days per week, daily operation from 06:00 to 18:00. Accordingly, the cumulative operating time for the whole year is 1862 hours. The following table lists heat recovery by month.

Heat recovery, kWh

Sensible

Latent

January

57.228,1

7.629,5

February

48.529,4

4.467,2

March

42.544,3

778,3

April

28.223,8

11,6

May

June

July

August

September

October

November

42.229,4

551,2

Dezember

54.283,6

4.877,2

Total for Year

273.038,6

18.314,9

By adding the amounts for sensible and latent heat recovery, we obtain a total energy recovery figure of 291,354 kilowatt-hours per year. Meanwhile, overall humidity recovery is 26,363 kg per year.

From these values we can finally determine the CO2 reduction potential from our first two tables.

It clearly emerges that: Anybody using heat exchangers in their air handling units can significantly reduce their heating/cooling costs, while also cutting emissions of the greenhouse gas carbon dioxide.